Any valve have a flow capacity depending not on the nominal diameter, but on the characteristics (size and form) of the 'light opening' through the valve.
As known this is normally given by the Cv (or European Kv) value. This and the pressure difference between inlet and outlet side of the valve gives the flow.
For nearby smaller sizes it is quite common that Cv's might vary and be larger for smaller valves, depending of the valves exact inner trim and form.
The difference might also stem from uncorrect or varying measurement of flow data at site.